With the continuous development of Ethernet techniques, an Ethernet Passive Coaxial Network (EPCN) system currently emerges. The EPCN system employs the Ethernet as transmission medium. FIG. 1 is a schematic diagram illustrating a structure of an EPCN system. As shown in FIG. 1, the EPCN system mainly includes a Coaxial-Cable Line Terminal (CLT), a branch/distributor (hereinafter called as branch) and a Coaxial-Cable Network Unit (CNU). In the EPCN system, a structure of point-to-multipoint is adopted, i.e., one CLT communicates with a plurality of CNUs through a passive coaxial cable. In the uplink direction, data streams are transmitted in form of burst Ethernet frames. The EPCN system is applicable for various service environments. For example, the most popular environment is an application in corridor for users of Ethernet broadband.
In the EPCN system, a basic data transmission procedure includes: in the downlink direction, the CLT transmits data to the branch, and the data are transmitted to the CNUs of different users through the branch. In the uplink direction, each CNU transmits data of itself to the branch, and the data are transmitted to the CLT through the branch.
It can be seen from the above data transmission procedure in the EPCN system that, in the uplink direction, the data of any CNU must pass the branch before arriving at the CLT. Therefore, in order to avoid that the data sent by one CNU destined at the CLT passes the branch and then arrives at another CNU, the branch has a fixed level isolating amplitude. The level isolating amplitude is higher than a transmission path loss between the CLT and the branch when data is transmitted. For example, the level isolating amplitude usually is 25 dBv. Thus, only if a transmission level adopted by the CNU when transmitting uplink data is higher than the sum of a receiving level of the CLT and the transmission path loss between the CLT and the branch but is lower than the level isolating amplitude of the branch, the branch is able to isolate the data, i.e., transmit the data only to the CLT but not to the CNU of another user.
However, in practical service implementations, the transmission levels adopted by all the CNUs are generally uniform, and the uniform transmission level is determined according to a maximum transmission level required by the CNUs. The maximum transmission level is generally higher than the level isolating amplitude of the branch. Thus, the branch can not isolate the data transmitted by the CNUs, which results in that the data transmitted by the CNU of one user is filched by the CNU of another user.
For example, take the scenario that the EPCN system is applied in corridor for users of Ethernet broadband as an example. Each CNU is equipped respectively in home of each user. The transmission path loss from the CLT to a TV signal access point of each home is basically the same. But since the CNUs in different homes have different locations, each home may have a different indoor transmission path loss. For example, the transmission path loss from the CLT to each access point is 22 dBv. A CNU1 of a user 1 is equipped at the access point, e.g. living room. Thus, the transmission path loss between the CNU1 of the user 1 and the CLT is 22 dBv. For a user 2, a CNU2 is equipped behind a 3-distributor the loss of which is generally 6 dBv, e.g. the CNU2 is equipped at a bedroom. Thus, the total transmission path loss between the CNU2 of the user 2 and the CLT is 22 dBv+6 dBv=28 dBv. In other words, the transmission path loss between the CNU1 and the CLT and that between the CNU2 and the CLT are different. Suppose the minimum receiving level of the CLT is 1 dBv. Since all CNUs will transmit data signals according to the maximum transmission path loss, both the CNU1 and the CNU2 will transmit the uplink data using a transmission level of 28 dBv+1 dBv. As to the CNU2 of the user 2, the transmission level of 28 dBv+1 dBv will become 22 dBv+1 dBv when the data signal arrives at the access point due to the loss of the 3-distributor in the user 2's home. The level isolating amplitude of the branch in corridor is 25 dBv. Thus, the branch is able to isolate the uplink data transmitted by the CNU2. Therefore, the uplink data transmitted by the CNU2 will not be filched by the CNU in other user's home. However, if the CNU1 also transmits uplink data by using the transmission level of 28 dBv+1 dBv, since there is no loss of 3-distributor in the user 1's home, the signal energy of 28 dBv+1 dBv will directly adds to the branch in corridor. Since the level isolating amplitude of the branch is 25 dBv, the branch cannot isolate the uplink data transmitted by the CNU1 of the user 1. Thus, the uplink data transmitted by the CNU1 may be filched by the CNU in other user's home.
It can be seen from the above that, in the prior art, the branch connected with different users cannot ensure the isolation of the uplink data transmitted by the CNUs in essence. Thus, the data transmitted by the CNU of one user may be filched by the CNU of another user, thereby dramatically decreasing the security of the data uplink transmission and decreasing service quality.